1. Field of the Invention
The present invention relates to a machine tool, and more specifically to a machine tool capable of detecting a change in ambient temperature of the machine tool and a temperature detector abnormality.
2. Description of Related Art
Compensation for thermal deformation of a machine tool, which measures the ambient temperature of the machine tool and compensates the machine origin position and the like according to the measured temperature, is widely carried out.
For example, the invention that measures the environmental temperature around the machine tool and the temperature of constituents forming the machine tool by using a temperature sensor, obtains a thermal deformation value on the basis of the difference between the measured temperatures and the reference temperature preliminarily obtained, and calculates a machine origin shift amount on the basis of the thermal deformation value has been well known (see JP 3-79256A).
There is another well-known invention that sets as reference temperature the temperature measured by a temperature sensor disposed in a bed in which there are not many changes in temperature, deducts the reference temperature from the temperature measured by a temperature sensor disposed in a heat-generating portion of a spindle or spindle head to obtain a temperature rise value, and adds a preset temperature shift value to the temperature rise value to calculate a shift temperature rise value, obtains a provisional compensation value from the shift temperature rise value and a temperature-compensation amount relationship table, and multiplies the provisional compensation amount by a compensation coefficient of the target spindle to obtain a thermal deformation compensation amount of the target spindle (see JP 10-6183A).
Furthermore, a device for calculating a thermal deformation amount of a machine tool, in which thermal events, such as room temperature change, operation heat, and machining heat, are measured to store the degree of the thermal events in addition to occurrence time, and a remaining thermal deformation amount is obtained from the stored data and an elapsed time coefficient preliminarily obtained by experiment or calculation, has been also well-known (see JP 2000-135654A).
Moreover, a recent motor (servomotor and spindle motor) is provided in its inside with a temperature detector for detecting the temperature of the motor itself and performing a temperature compensation with respect to a secondary resistance value. As a heating value caused by motor drive current is proportional to the square of a feedback value of the drive current, the heating value can be calculated from the current feedback amount. Additionally, it is known that the relationship between the heating value and the motor temperature is preliminarily obtained, and the motor temperature can be calculated and estimated from the heating value calculated on the basis of this relationship formula (see JP 7-59399A).
As described above, in conventional machine tools, the temperature detector is disposed in the inside or outside of each of the machine tools, to thereby measure the ambient temperature. Depending on a surrounding environment including the position, space or the like where the temperature detector is installed, a countermeasure, such as addition of a cover, is required in order to protect the temperature detector against a coolant, swarf and the like, which increases costs.